Documentation and Comments

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Documenting and Comments
Comments that contradict the code are worse than no comments. Always make a priority of keeping the comments up-to-date when the code changes!
Comments should be in English, and should be complete sentences. If the comment is short, the period at the end can be omitted.
Remember that comments should serve to your reasoning and decision-making, not attempt to explain what a command does. With the exception of regular expressions, well-written PowerShell can be pretty self-explanatory.
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# Do not write:
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# Increment Margin by 2
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$Margin = $Margin + 2
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​
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# Maybe write:
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# The rendering box obscures a couple of pixels.
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$Margin = $Margin + 2
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Block comments
Don't go overboard with comments. Unless your code is particularly obscure, don't precede each line with a comment -- doing so breaks up the code and makes it harder to read. Instead, write a single block comment.
Block comments generally apply to some or all of the code which follows them, and are indented to the same level as that code. Each line should start with a # and a single space.
If the block is particularly long (as in the case of documentation text) it is recommended to use the <# ... #> block comment syntax, but you should place the comment characters on their own lines, and indent the comment:
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# Requiring a space makes things legible and prevents confusion.
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# Writing comments one-per line makes them stand out more in the console.
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​
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<#
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    .SYNOPSIS
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        Really long comment blocks are tedious to keep commented in single-line mode.
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    .DESCRIPTION
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        Particularly when the comment must be frequently edited,
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        as with the help and documentation for a function or script.
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#>
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Inline comments
Comments on the same line as a statement can be distracting, but when they don't state the obvious, and particularly when you have several short lines of code which need explaining, they can be useful.
They should be separated from the code statement by at least two spaces, and ideally, they should line up with any other inline comments in the same block of code.
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$Options = @{
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    Margin = 2          # The rendering box obscures a couple of pixels.
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    Padding = 2         # We need space between the border and the text.
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    FontSize = 24       # Keep this above 16 so it's readable in presentations.
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}
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Documentation comments
Comment-based help should be written in simple language.
You're not writing a thesis for your college Technical Writing class - you're writing something that describes how a function works. Avoid unnecessarily large words, and keep your explanations short. You're not trying to impress anyone, and the only people who will ever read this are just trying to figure out how to use the function.
If you're writing in what is, for you, a foreign language, simpler words and simpler sentence structures are better, and more likely to make sense to a native reader.
Be complete, but be concise.
Location
In order to ensure that the documentation stays with the function, documentation comments should be placed INSIDE the function, rather than above. To make it harder to forget to update them when changing a function, you should keep them at the top of the function, rather than at the bottom.
Of course, that's not to say that putting them elsewhere is wrong -- but this is easier to do, and harder to forget to update.
Put Details in the Notes
If you want to provide detailed explanations about how your tool works, use the Notes section for that.
Describe The Function
Every script function command should have at least a short statement describing it's function. That is the Synopsis.
Document Each Parameter
Each parameter should be documented. To make it easier to keep the comments synchronized with changes to the parameters, the preferred location for parameter documentation comments is within the param block, directly above each parameter. Examples can be found in the ISE snippets:
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param (
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    # Param1 help description
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    [Parameter(Mandatory = $true,
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                ValueFromPipelineByPropertyName = $true,
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                Position = 0)]
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    $Param1,
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​
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    # Param2 help description
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    [int]
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    $Param2
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)
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It is also possible to write .PARAMETER statements with the rest of the documentation comments, but experience shows they are more likely to be kept up-to-date if you put them closer to the code they document.
Provide Usage Examples
Your help should always provide an example for each major use case. A 'usage example' is just an example of what you would type in to Powershell to run the script - you can even cut and paste one from the command line while you're testing your function.
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function Test-Help {
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    <#
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        .SYNOPSIS
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            An example function to display how help should be written.
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​
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        .EXAMPLE
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            Get-Help -Name Test-Help
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​
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            This shows the help for the example function.
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    #>
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    [CmdletBinding()]
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    param (
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        # This parameter doesn't do anything.
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        # Aliases: MP
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        [Parameter(Mandatory = $true)]
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        [Alias("MP")]
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        [String]$MandatoryParameter
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    )
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​
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    <# code here ... #>
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}
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DOC-01 Write comment-based help
You should always write comment-based help in your scripts and functions.
Comment-based help is formatted as follows:
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function Get-Example {
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    <#
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    .SYNOPSIS
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        A brief description of the function or script.
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​
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    .DESCRIPTION
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        A longer description.
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​
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    .PARAMETER FirstParameter
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        Description of each of the parameters.
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        Note:
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        To make it easier to keep the comments synchronized with changes to the parameters,
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        the preferred location for parameter documentation comments is not here,
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        but within the param block, directly above each parameter.
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​
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    .PARAMETER SecondParameter
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        Description of each of the parameters.
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​
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    .INPUTS
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        Description of objects that can be piped to the script.
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​
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    .OUTPUTS
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        Description of objects that are output by the script.
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​
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    .EXAMPLE
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        Example of how to run the script.
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​
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    .LINK
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        Links to further documentation.
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​
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    .NOTES
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        Detail on what the script does, if this is needed.
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​
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    #>
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Comment-based help is displayed when the user types help Get-Example or Get-Example -?, etc.
Your help should be helpful. That is, if you've written a tool called Get-LOBAppUser, don't write help that merely says, "Gets LOB App Users." Duh.
Further information: You can get more on the use of comment-based help by typing help about_Comment_Based_Help within Powershell.